Abstract: Detecting transient phase shifts in an object laser beam of predetermined frequency having an arbitrary wavefront and apparatus for directing said object laser beam to cross a second reference laser beam coherent with said object beam into an oriented photorefractive crystal (22) (Bi12TiO20) (applied alternating electric field) belonging to the crystal symmetry group of 43m or 23, setting the polarization state of said object beam (20, 26) and said reference beams (32, 34) to be elliptical and different one from other, causing said object laser beam to interfere with said reference beam inside the said photorefractive crystal so as to form a dielectric-permittivity-tensor grating diffracting said reference beam into said object beam and vice versa, and directing a beam transmitted through the said photorefractive crystal in the direction of a transmitted object beam onto a photo-detector (44) to result in an electrical output signal that is representative of the transient phase shift in the object laser beam.